In fossil-fueled power stations for generating electric energy, a carbon dioxide-containing off-gas is produced as a result of combusting a fossil fuel. This carbon dioxide-containing off-gas as a rule is released into the atmosphere. In order to achieve a reduction of the carbon dioxide emission in fossil-fueled power stations, carbon dioxide can be separated from the off-gas.
For separating carbon dioxide from a gas mixture, different methods are generally known. In particular, for separating carbon dioxide from an off-gas after a combustion process, the method of absorption-desorption is in common use. On a commercial scale, the described separation of carbon dioxide with the absorption-desorption method is carried out with a selective scrubbing agent for carbon dioxide. In this case, the off-gas in an absorption column is brought into contact with the selective solvent, wherein carbon dioxide is bound in the solvent. The off-gas which is largely cleaned of carbon dioxide is released from the absorption column for a further process or for discharging. The solvent which is laden with carbon dioxide is directed into a desorption column for separation of the carbon dioxide and regeneration of the solvent. The separation in the desorption column can be carried out thermally. In this case, the laden solvent is heated, wherein a gas-vapor mixture consisting of gaseous carbon dioxide and evaporated solvent is produced—this being the so-called exhaust vapor. The evaporated solvent is then separated from carbon dioxide. The separated carbon dioxide can now be compressed and cooled in a plurality of stages. In liquid or frozen state, carbon dioxide can then be fed to a storage facility or to a further application. The regenerated solvent is directed once more to the absorption column where carbon dioxide can again be absorbed from the carbon dioxide-containing off-gas.
A great disadvantage of the absorption-desorption method above all is that for desorption a great deal of energy generally has to be expended. This energy as a rule is extracted in the form of heating steam from the power station process, which significantly impairs the overall efficiency of the power station. In order to reduce the necessary energy expenditure for the desorption, the prior art already discloses a series of improvement proposals, wherein the energy expenditure is to be optimized particularly by means of an improved integration of the absorption-desorption process into the power station process.
A big problem, furthermore, is that the described absorption-desorption process above all is very slow-acting on account of the necessary constructional size of the absorption-desorption column. A largely comprehensive separation of carbon dioxide from an off-gas of the power station can only effectively begin, moreover, when heating steam can adequately be made available by means of the power station and the desorption column is adequately heated through. Up to this point in time, it was previously possible for large amounts of carbon dioxide-laden off-gas to be released into the atmosphere in an uncleaned state.
A power station, for example a combined cycle power station, but also increasingly steam power stations, are taken off the grid with increasing frequency, for example each night, or each weekend. With the power station shut down, there is no longer the development of carbon dioxide-containing flue gas. However, steam or heating steam is no longer available either. If the carbon dioxide separation apparatus is shut down together with the power station without additional measures, a series of problems can occur. Thus, the carbon dioxide-laden solvent which remains in the absorption-desorption apparatus cools down, as a result of which solubility limits are fallen short of, and precipitation and sedimentation of products can occur. As a result of settling particles or suspended particles, which are contained within the solvent, there is an increased risk of blocking.
Shutting down a carbon dioxide separation apparatus which is integrated into a power station also proves to be difficult. During a planned downtime of the absorption-desorption plant, the solvent which is contained within the entire solvent circuit has to be largely completely desorbed for avoiding the risk of blocking. For this, the absorption column is separated from the off-gas stream and the desorption column is heated in addition. As a result, further carbon dioxide is desorbed by means of the desorption column and further carbon dioxide is no longer absorbed in the absorption column. After a comparatively long time, the solvent is regenerated to the point where the station can be shut down. Up to that point, the carbon dioxide separation apparatus has to be supplied with sufficiently high-quality heating steam. If the power station is to be started up again, the solvent has to be heated again. Until reaching the operating temperature, a comparatively long time elapses in which no carbon dioxide can be separated or an economical degree of separation is not achieved.
The largely complete desorption of the solvent for a short downtime, however, on the one hand is uneconomical, and, moreover, is associated with a long restarting time of the desorption column. Therefore, the carbon dioxide separation apparatus is preferably kept in standby mode in the case of a short downtime. In standby mode, however, heating of the solvent is furthermore necessary in order to ensure a fast restart on the one hand and on the other hand to prevent a possible crystallization or precipitation of the solvent. Moreover, it is also necessary in standby mode that the solvent is additionally circulated, that is to say is pumped round the circuit. As a result of the circulation, the occurrence of products of humidification or evaporation as a result of retention of the solvent in any fillets or dead spaces, which again lead to crystallization, is prevented. A reduction of the flow rate in standby mode is certainly possible, but only providing the entire solvent circuit is adequately exposed to circulation at each point.